Method and apparatus for detecting abnormal combustion conditions in reciprocating engines having high exhaust gas recirculation

ABSTRACT

An apparatus and method to detect abnormal combustion conditions for use as a feedback control of an EGR (Exhaust Gas Recirculation) controlled reciprocating engine using ionization signals is presented. The system receives a succession of ionization signals for successive cycles of a running engine and processes a plurality of related ionization signals for signal stability. The ionization signals are checked to determine if an abnormal combustion condition such as knock or misfire has occurred. The variation of an ionization signal that changes with respect to an engine parameter over a combustion event of the reciprocating engine is measured and a floating bounded space is associated with the ionization signal. An indication that the abnormal combustion condition has been detected is provided if a portion of the ionization signal is within the floating bounded space.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 10/286,353, filed Nov. 1, 2002 now U.S. Pat. No.6,742,499.

FIELD OF THE INVENTION

The present invention relates generally to ignition systems in sparkignited engines, and more particularly relates to such systems inreciprocating engines using exhaust gas recirculation.

BACKGROUND OF THE INVENTION

Industry has developed various techniques using ionization signals fordetecting abnormal combustion conditions such as misfire, knock, andapproximate air/fuel ratio for stochiometric engines. Free ions presentin the combustion gases are electrically conductive and are measurableby applying a voltage across an ionization probe. Alternatively, thevoltage is applied across the electrodes of a spark plug after the sparkplug has ignited the combustion mixture. The applied voltage induces acurrent in the ionized gases which is measured to provide the ionizationsignal. The ionization signal is used as a control parameter in thecontrol of the engine. For example, in U.S. Pat. No. 6,029,627,ionization signals and a single O₂ sensor in the exhaust are used tocontrol the air/fuel ratio in engines to achieve stoichiometricoperation. This technique uses the O₂ sensor to achieve stoichiometry ofthe overall stoichiometric mixture of the engine and then equalizes theamplitude or location of the first local peak of the ionization signalin each individual cylinder. Another technique disclosed in U.S. Pat.No. 5,992,386 performs a frequency analysis of the ionization signal todetect abnormal combustion conditions such as knock. These systemsintegrate the ionization signal and compare the magnitude of theintegrated signal to the magnitude of the integrated signal of a normalcombustion event. The abnormal combustion condition is detected if themagnitude of the integrated signal is above a threshold level, which isset above the magnitude of the integrated signal of a normal combustionevent.

One of the drawbacks of stochiometric engines is the emission ofpollutants. With fixed engine timing and load, the NO_(x) emissionslevel of a typical gas engine is dependent upon the air/fuel ratio. Neara chemically correct (i.e., stoichiometric) ratio, the NO_(x) emissionspeak and then drop significantly as the amount of excess air isincreased. Maintaining a stable combustion process with a high air/fuelratio is difficult to manage. As a result, conventional spark-ignitedgas engines typically operate near the stoichiometric air/fuel ratio anddepend upon exhaust after treatment with catalytic converters to reducethe NO_(x) emissions.

Government agencies and industry standard setting groups are reducingthe amount of allowed emissions in an effort to reduce pollutants. As aresult, industry is moving towards using lean burning engines to reduceemissions despite the difficulty of maintaining a stable combustionprocess in lean burning engines. By using more air during combustion,turbocharged lean-burn engines can enhance fuel efficiency withoutsacrificing power and produce less nitrous oxide pollutants thanconventional stoichiometric engines.

Another method to reduce emissions is to add exhaust gas to thecombustion mixture instead of more air to reduce emissions (i.e.,exhaust gas recirculation). Exhaust gases have already combusted, sothey do not burn again when they are recirculated. Since the exhaust gasdoes not burn, it lowers the peak combustion temperature. At lowercombustion temperatures, nitrogen cannot form compounds with oxygen andis carried out of the system with the exhaust gas. NOx controlrequirements vary on various engines and so there are various controlsystems to provide these functions. The most commonly used in mostmodern day cars is the vacuum-operated EGR valve. There are also othercontrols that relate directly to the EGR systems and which complete thesame function within the system like the thermal vacuum switch, variablevalve timing, ported vacuum switch, venturi vacuum amplifier, EGR delaytimer control, back pressure transducer, etc. The amount of EGR gas flowfor each engine has to be calibrated as too much or too little canhamper performance by changing the engine breathing characteristics andmay cause misfire or knock. Too much exhaust gas flow will retard engineperformance and may cause misfire. Too little exhaust gas flow willincrease NOx and may cause engine knock.

Ionization sensing has not been utilized to any significant extent inthese EGR controlled engines. Because of the nature of the mixture, theionized species concentration, including NO_(x), is much less than atstoichiometric conditions. As a result, the ionization signal is of verylow intensity and has great variability. The techniques developed usingionization signals for stochiometric operation are unsuitable for highEGR operation. For example, the ionization signals of some high EGRoperated engines are sufficiently variable and are low enough inmagnitude that integrating the signal can not be done reliably due to anumber of factors. These factors include higher levels of noise relativeto the ionization signal magnitude, the variability of the ionizationsignal, and the low magnitudes of the resultant integrated signal.

BRIEF SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toreliably detect abnormal combustion conditions such as misfire and knockof engines based on ionization signals for use with Exhaust GasRecirculation (EGR) control of stochiometric engines.

The foregoing objects are among those attained by the invention, whichprovides a method of detecting an abnormal combustion condition in acombustion chamber of a reciprocating engine. The abnormal combustioncondition includes misfire and knock. The method measures the variationof an ionization signal that changes with respect to an engine parameterover a combustion event of the reciprocating engine, associates afloating bounded space with the ionization signal, determines if aportion of the ionization signal is within the floating bounded space,and provides an indication that the abnormal combustion condition hasbeen detected if the portion of the ionization signal is within thefloating bounded space.

A method to determine the floating bounded space and a starting pointfor the floating bounded space is also disclosed. The method includesreceiving a set of ionization signals that change with respect to anengine parameter over a combustion event. The set of ionization signalshas ionization signals corresponding to normal combustion conditions andionization signals corresponding to at least one abnormal combustioncondition for an engine operating with high EGR. The method furtherincludes the step of adjusting the starting point and size of thefloating bounded space such that selected portions of the ionizationsignals corresponding to the at least one abnormal combustion conditionreliably fall within the floating bounded space and the ionizationsignals corresponding to normal combustion conditions reliably falloutside the floating bounded space.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thespecification illustrate several aspects of the present invention, andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 a is a schematic view of an air/fuel ratio using EGR control ofthe present invention;

FIG. 1 b is a block diagram of the ionization module of FIG. 1 a;

FIG. 2 is a flow chart illustrating the steps to characterize an engineand determine parameters of a floating bounded space in accordance withthe teachings of the invention;

FIG. 3 a is a graphical representation of pressure and ionizationcurrent versus engine piston crank angle for a normal combustion event;

FIG. 3 b is a graphical representation of pressure and ionizationcurrent versus engine piston crank angle for a misfire event;

FIG. 3 c is a graphical illustration of experimental data showing acorrelation between indicated mean effective pressure of an enginecylinder and misfire that is used in sizing the floating bounded spaceof the present invention;

FIG. 4 a is a graphical representation of pressure and ionizationcurrent versus engine piston crank angle for a normal combustion eventof a spark plug design having a high electrode surface area andelectrodes that are mostly exposed to combustion chamber air flow;

FIG. 4 b is a graphical representation of pressure and ionizationcurrent versus engine piston crank angle for an incipient knock event ofthe spark plug design of FIG. 4 a;

FIG. 4 c is a graphical representation of pressure and ionizationcurrent versus engine piston crank angle for a severe knock event of thespark plug design of FIG. 4 a;

FIG. 4 d is a graphical illustration of experimental data showing acorrelation between the peak of the derivative of pressure of an enginecylinder as a function of crank angle and incipient knock and severeknock of the spark plug design of FIG. 4 a that is used in sizing thefloating bounded space of the present invention;

FIG. 5 a is a graphical representation of pressure and ionizationcurrent versus engine piston crank angle for a normal combustion eventof a spark plug design having a high electrode surface area andelectrodes that are mostly shielded from combustion chamber air flow;

FIG. 5 b is a graphical representation of pressure and ionizationcurrent versus engine piston crank angle for an incipient knock event ofthe spark plug design of FIG. 5 a;

FIG. 5 c is a graphical representation of pressure and ionizationcurrent versus engine piston crank angle for a severe knock event of thespark plug design of FIG. 5 a;

FIG. 5 d is a graphical illustration of experimental data showing acorrelation between the peak of the derivative of pressure of an enginecylinder as a function of crank angle and incipient knock and severeknock of the spark plug design of FIG. 5 a that is used in sizing thefloating bounded space of the present invention;

FIG. 6 is a flow chart illustrating the steps to determine an abnormalcombustion condition in accordance with the teachings of the presentinvention;

FIG. 7 is a flow chart illustrating the steps to determine the abnormalcombustion condition of FIG. 6; and

FIG. 8 is a schematic view illustrating the use of a secondary source inaccordance with the teachings of the present invention.

While the invention will be described in connection with certainpreferred embodiments, there is no intent to limit it to thoseembodiments. On the contrary, the intent is to cover all alternatives,modifications and equivalents as included within the spirit and scope ofthe invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an apparatus and method to detectabnormal combustion conditions in a high EGR controlled reciprocatingengine using ionization signals. As used herein, high EGR means a levelof EGR such that NO_(x) is within desired levels. Referring initially toFIG. 1 a, a system 100 exemplifying the operating environment of thepresent invention is shown. The system includes an ionization module102, an air/fuel module 104, a spark module 106, an EGR module 130 and areciprocating engine. While the ionization module 102, the air/fuelmodule 104, the spark module 106 and the EGR module 130 are shownseparately, it is recognized that the modules 102, 104, 106, 130 may becombined into a single module or be part of an engine controller havingother inputs and outputs. The reciprocating engine includes enginecylinder 108, a piston 110, an intake valve 112 and an exhaust valve114. An intake manifold 116 is in communication with the cylinder 108through the intake valve 112. An exhaust manifold 118 receives exhaustgases from the cylinder 108 via the exhaust valve 114. The intake valve112 and exhaust valve 114 may be electronically, mechanically,hydraulically, or pneumatically controlled or controlled via a camshaft.A spark plug 120 with a spark gap 122 ignites the air/fuel mixture incylinder 108. Spark module 106 controls ignition timing and providespower to the spark plug 120. The exhaust manifold 118 is in fluidcommunication with EGR valve 128. The EGR valve 128 provides exhaust gasto the intake manifold 116, preferably downstream of the throttle valve126. For simplicity, the recirculation path from the EGR valve 128 tothe intake is designated by arrows 132. In some systems, the exhaust gasmay be further cooled by means of a cooler in the exhaust gasrecirculation path. Additionally, the exhaust valve 114 can becontrolled with variable timing to assist in keeping some of the exhaustgas in the cylinder 108.

The ionization module contains circuitry for detecting and analyzing theionization signal. In the illustrated embodiment, as shown in FIG. 1 b,the ionization module includes an ionization signal detection module140, an ionization signal analyzer 142, and an ionization signal controlmodule 144. In order to detect abnormal combustion conditions, theionization module 102 supplies power to the spark gap 122 after the airand fuel mixture is ignited and measures ionization signals from thespark gap 122 via ionization signal detection module 140. Alternativelya conventional ionization probe or other conventional device to detectionization may be used to measure the ionization signals. Ionizationsignal analyzer 142 receives the ionization signal from ionizationsignal detection module 140 and determines if an abnormal combustioncondition exists. The ionization signal control module 144 controlsionization signal analyzer 142 and ionization signal detection module140. The ionization signal control module 144 provides an indication tothe air/fuel module 104, spark module 106, and EGR module 130 of theabnormal combustion condition as described below. In one embodiment, theionization module 102 sends the indication to other modules in theengine system such as an engine controller. While the ionization signaldetection module 140, the ionization signal analyzer 142, and theionization signal control module 144 are shown separately, it isrecognized that they may be combined into a single module and/or be partof an engine controller having other inputs and outputs.

Returning now to FIG. 1 a, the air/fuel module 104 controls fuelinjection 124 and may control throttle valve 126 to deliver air andfuel, at a desired ratio, to the engine cylinder 108. The air/fuelmodule 104 receives feedback from the ionization module and adjusts theair/fuel ratio as described below. The EGR module 130 controls theamount of exhaust gas recirculated into the intake manifold andtherefore into the cylinder.

In the description that follows, the “air” in the air/fuel ratioincludes exhaust gas recirculated into the cylinder. The ionizationsignal is proportional to the air/fuel ratio of the fuel mixture. Theair/fuel ratio of the mixture is higher in lean burn engines (i.e., theamount of fuel is lower) than in stochiometric engines. The lower amountof fuel relative to air results in a lower flame temperature, whichtranslates into a lower number of free ions present in the combustiongases. In addition, the spark plug design in conjunction with thegasdynamic and thermodynamic characteristics of the combustion eventgreatly affect the magnitude and repeatability of ion signal. Forexample, systems having spark plugs having a high electrode surface areaand electrodes that are mostly shielded from the combustion chamber airflow provide higher magnitude and more consistent ionization signalsthan other types of spark plugs. On the other hand, the ionizationsignal is not easy to detect or process in lean burn engines usingconventional “J-gap” automotive type spark plugs because the signal isof very low intensity and has great variability. Prior art systems thatuse the energy delivered to ignite the fuel mixture to detect theionization signal will not work properly because these system will get aweak signal or no signal at all. The preferred form of the presentinvention supplies power to the spark gap after the air and fuel isignited to measure ionization signals. Additional free ions flow whenthe additional power is applied, thereby resulting in an ionizationsignal that is easier to detect.

The ionization signal is acquired with respect to an engine parameterover the combustion cycle. For example, the engine parameter may becrank angle, time after ignition, time from top dead center, etc. Crankangle is used herein in its most generic sense to include all of these.For example, crank angle is intended to be generic to measurement of theengine rotational parameter no matter whether it is measured directly interms of crank angle degrees, or measured indirectly or inferred bymeasurement. It may be specified with respect to top dead center, withrespect to ignition point, etc. Abnormal combustion conditions such asmisfire and knock are detected at specific points in the combustioncycle. These points are where the ionization signal of the abnormalcombustion condition has a signal characteristic that is different fromionization signals of normal combustion conditions. For example, misfireoccurs when the ionization signal remains at or near an initial valuefor an extended interval of the combustion cycle. A misfire condition isoften due to an inadequate air/fuel ratio (e.g., too lean), sparktiming, and/or spark characteristics.

In order to detect abnormal combustion events in an EGR controlledengine, a floating bounded space is associated with the ionizationsignal (measured with respect to a combustion event) to detect theabnormal combustion conditions. The floating bounded space is a spacethat is located at a position in the combustion cycle and sized suchthat a portion of the ionization signal will reliably be within thespace during the abnormal combustion condition and reliably be outsidethe space during normal combustion conditions. The position of thefloating bounded space is a function of an engine timing parameter(e.g., crank angle, time, etc.) and the size is a function of the enginetiming parameter and ionization signal magnitude. For example, afloating bounded space shaped as a box has one axis (e.g., length) ofthe box in units of the engine parameter (e.g., crank angle) and theother axis (e.g., height) of the box in units of ionization signalmagnitude. Preferably, a floating bounded space is used for eachabnormal combustion condition (e.g., a floating bounded space formisfire, a floating bounded space for incipient knock, a floatingbounded space for severe knock, etc.).

When the ionization signal reaches the point in the combustion cyclewhere the floating bounded space has been positioned, the ionizationsignal magnitude is compared to the magnitude range of the floatingbounded space. The ionization module 102 indicates that an abnormalcombustion condition has occurred if the ionization signal is within thefloating bounded space according to criteria described below. Use of thefloating bounded space according to the invention overcomes the problemsassociated with prior art integrating techniques. The effect of noise isreduced by eliminating the integration of the ionization signal.Integration is implicitly a filtering operation that can miss shortbursts of activity that are indicative of combustion conditions. In leanburn combustion these short bursts may be the only difference between anormal combustion condition and an abnormal combustion condition. Thefloating bounded space detects the short bursts. The variability in theionization signal is accounted for during a calibration process when thefloating bounded space is sized and positioned as described herein.

Signal characteristics of the ionization signal are used to characterizeeach abnormal combustion condition and to determine at what point in thecombustion cycle that the abnormal combustion condition can be reliablydetected. The floating bounded space is derived to capture the signalcharacteristic of the ionization signal indicative of the abnormalcombustion condition and is associated with the ionization signal. Inthe description that follows, the details of determining the startingposition and the size of the floating bounded space will be discussedand then the details of detecting an abnormal combustion condition willbe described. A floating box shall be used to describe positioning andsizing the floating bounded space. The ionization signal will beacquired with respect to crank angle. It is recognized that any shapemay be used for the floating bounded space and the ionization signal canbe acquired with respect to other engine parameters.

Turning now to FIG. 2, the overall steps of the tuning process areshown. The tuning process determines the starting position and the sizeof the floating bounded space. The process also determines an air/fuelratio limit for a given spark characteristic and spark timing at whichthe engine has a high likelihood of misfire if the engine is operatingwith an air/fuel ratio above the air/fuel ratio limit. As the air/fuelratio becomes leaner (e.g., too much exhaust gas is recirculated), theprobability of misfire increases. The air/fuel ratio limit and EGR limitare set based upon operating constraints. For example, an engine may beallowed to misfire a percentage of the number of cycles during operationwhile another engine may never be allowed to misfire. The limit is setto a richer air/fuel ratio if the engine is not allowed to misfire thanthe air/fuel ratio of an engine that is allowed to misfire duringoperation. Although air/fuel ratio, percent EGR and spark timing are keycontrol parameters, it is recognized that other engine parameters can beused to control an engine (e.g., waste gate and throttle position,etc.). A set of data points of engine parameters that can be used todetermine the abnormal combustion condition and a set of correspondingionization signals at various operating conditions of the lean burnengine is obtained (step 200). The set of data points of engineparameters may be indicated mean effective pressure (IMEP) of thecylinder, air/fuel ratio, or any other engine parameter that can be usedto determine when the abnormal combustion condition has occurred. A testengine is typically used to obtain the set of data points and learn thecharacteristics of the engine during normal and abnormal operatingconditions. The use of a test engine allows sensors and diagnosticequipment to be used that are typically not available in productionengines. For example, the IMEP of a cylinder is generally not acquirablein production engines from cylinder pressure sensors because productionengines generally do not have pressure sensors in each cylinder due tocost and reliability issues. In some lean burn systems, the ionizationsignal may be noisy. In these systems, the set of correspondingionization signals are acquired using filters to filter the ionizationsignal. For example, a moving average filter can be used where thenumber of data points to average is defined based upon signalcharacteristics of the ionization signal.

The abnormal combustion condition is determined from the set of datapoints (step 202). For example, a misfire can be detected using the IMEPof a cylinder. A misfire occurs if the IMEP is below a definedthreshold. In one embodiment this threshold is a predeterminedpercentage of the nominal value of IMEP for the cylinder. The ionizationsignals corresponding to the abnormal combustion condition are comparedto the ionization signals of normal combustion conditions to determinecharacteristics of the ionization signal that can be used to identifythe abnormal event (step 204). The starting point and size of thefloating box is then determined using the characteristics of theionization signals (step 206). In one embodiment, the starting point andsize is determined by looking at the abnormal combustion conditions anddetermining the upper and lower extremes in the data set. The floatingbox is sized and positioned at one extreme and then the floating box istuned at the other extreme. The floating box is preferably sized andpositioned with sets of data points acquired at different operatingconditions. The floating box parameters (i.e., size and position) mayvary with engine operating conditions, such as speed, engine load, anddesired air/fuel ratio. For example, the size of the floating box isdifferent at engine idle and full power.

Turning now to FIGS. 3 a-3 c, the floating box 300 for a misfire eventis shown. FIG. 3 a is an illustration of a representative cylinderpressure 302 and ionization signal 304 of a normal combustion condition.FIG. 3 b is an illustration of a representative cylinder pressure 306and ionization signal 308 for a misfire condition. A representative setof data points of the engine parameter for 70 engine cycles is shown inFIG. 3 c. The engine parameter used is the IMEP of a cylinder. If theIMEP of any data point is below a selected amount, the data point isclassified as a misfire condition. The selected amount should be set toa point that detects all the misfires. In one embodiment, the selectedamount is a predetermined percentage of nominal. Data points 310 in FIG.3 c correspond to a misfire condition. It can be seen that theionization signal 304 of a normal combustion condition has an initialshort flattened portion from the initial starting point followed by apeaked portion. In contrast, the misfire condition remains substantiallyconstant for a given duration. One characteristic of a misfire conditionin the ionization signal for many engines is that a portion of theionization signal remains substantially constant from the initialstarting point 312 of the ionization signal for an extended interval ascan be seen in FIG. 3 b and can be confined within a bounded space. Itis recognized that other characteristics may be used.

The tuning process is used to determine the starting point and size ofthe floating box using the characteristics of the ionization signals.The tuning process adjusts the size and position of the floating box toreliably capture the misfire condition and exclude the normal combustioncondition. The starting point and size of the floating box is adjusteduntil the floating box is of sufficient size and at a location of theionization signal with respect to crank angle such that a portion of theionization signal of a misfire condition reliably remains within thefloating box 300 for the duration of the floating box 300 as shown inFIG. 3 b and leaves the floating box 300 for a normal combustioncondition as shown in FIG. 3 a. This is accomplished by overlaying thefloating box on the ionization signals corresponding to the normal andabnormal combustion cycles shown in FIG. 3 c and adjusting the boxparameters (e.g., starting point (with respect to crank angle (i.e.,time) and ionization signal magnitude), duration, and height) tooptimize the box. For example, the floating box is superimposed onionization signals corresponding to the upper and lower extremes of datapoints 310 (i.e., the misfire conditions) in the engine beingcharacterized and the box parameters are adjusted such that the portionof the ionization signal reliably remains within the box for eachcondition. The floating box is then superimposed on the ionizationsignal for the normal ionization signals that are closest in form to theionization signals for misfire conditions. For example, the ionizationsignals corresponding to data points 312, 314, and 316 are likely to beclosest in shape or form to ionization signals corresponding to misfireconditions. The floating box is then adjusted until the portion of theionization signal of the normal combustion condition is not captured bythe floating box. This process is repeated for all of the ionizationsignals in the data set for the various engine operating conditions(e.g., speed, engine load, desired air/fuel ratio, etc.) to ensure thatthe floating box reliably captures misfire conditions and excludes otherconditions. The box parameters are then used during engine operation todetect misfire conditions.

During operation, the ionization signal analyzer 132 receives theionization signal. It floats the floating box over the ionization signalin accordance with the box parameters. In one embodiment, the lowestmagnitude of the ionization signal is determined beginning at thestarting point of the floating box and ending at the boundary of thefloating box (i.e., for the duration of the floating box). For example,if the duration of the floating box is thirty degrees of crank angle,the lowest magnitude of the ionization signal is determined over thethirty degrees of crank angle. The starting point of the floating box isthen positioned at the starting point crank angle (i.e., time afterignition) at the lowest magnitude of the ionization signal. Theionization signal analyzer 132 then determines if the ionization signalremains within the floating box over the duration of the floating box.The ionization signal analyzer 132 provides an indication to theionization signal control module 134 that a misfire has been detected ifthe ionization signal remains within the floating box over the durationof the floating box. FIG. 3 b illustrates the ionization signalremaining within the floating box over the duration of the floating box.

The ionization signal control module 134 provides an indication to theair/fuel module 104 spark module 106, and EGR module 130 of the misfirecondition and to other modules such as the engine controller. Theair/fuel module 104, EGR module 130, and spark module 106 (or the enginecontroller), in turn, determine what action to take. The actions thatcan be taken include reducing the amount of exhaust gas beingrecirculated, advancing the ignition timing and/or running the enginericher (e.g., adding more fuel to the air/fuel mixture) or doing nothinguntil a predetermined number of misfires have occurred and thenadvancing the ignition timing and/or running the engine richer. Theair/fuel module 104 controls fuel injection 124 and/or throttle valve126, the EGR module 130 controls the EGR valve 128, and spark module 106controls the spark timing to move the engine away from the misfirecondition in accordance with the action decided to be taken (e.g.,advancing the ignition timing and/or running the engine richer).

The ionization signals are substantially different in overall form orshape for different types of plugs. For example, the ionization signalmay have a secondary peak in some spark plugs and has no secondary peakin other types of spark plugs. This means that prior methods used todetect knock based on the presence of a secondary peak in the ionizationsignal will not work with certain types of spark plugs. The presentinvention is adaptable to many or most such types of spark plugs in thatthe engine is characterized with the spark plug types that are used inproduction engines. Turning now to FIGS. 4 a-4 d, the floating boundedspace 400 for knock is shown for a spark plug having a secondary peak.The onset of the second peak is an indication of knock. The floatingbounded space 400 is sized, positioned and in this case subdivided toinclude a lower portion 402 and an upper portion 404 to detect incipientknock and severe knock respectively. Incipient knock occurs when themagnitude of knock is minimal and the knock won't cause immediate damageto the engine. Severe knock occurs when the magnitude of the knock issuch that the knock is causing or is about to cause damage to theengine. FIG. 4 a is an illustration of a representative cylinderpressure 406 and ionization signal 408 of a normal combustion condition.FIG. 4 b is an illustration of a representative cylinder pressure 410and ionization signal 412 of an incipient knock condition. FIG. 4 c isan illustration of a representative cylinder pressure 414 and ionizationsignal 416 for a severe knock condition. Knock occurs when pressurechanges quickly. As a result, the engine parameter selected forcharacterizing the ionization signal is the peak of the derivative ofpressure of a cylinder with respect to engine crank angle. Other engineparameters could be used. A representative set of data points of theengine parameter is shown in FIG. 4 d. Acceptable knock 420, incipientknock 422, and severe knock 424 levels are shown.

A threshold level of the peak of the derivative of pressure of acylinder is chosen for incipient knock and a higher threshold level ofthe peak of the derivative of pressure of a cylinder is chosen forsevere knock. The threshold level for incipient knock is chosen suchthat the knock won't cause immediate damage to the engine. The thresholdlevel for severe knock is chosen such that the knock is about to do somedamage to the engine. If the peak of the derivative of pressure of acylinder is below the threshold level for incipient knock, any knockpresent is within an acceptable level of knock. If the peak of thederivative of pressure of a cylinder is above the threshold level forincipient knock and below the threshold for severe knock, the knock isdefined as incipient knock. If the peak of the derivative of pressure ofa cylinder is above the threshold level for severe knock, the knock isdefined as severe knock. For purposes of illustration, the thresholdlevel for incipient knock is set to a value of 25 and the thresholdlevel for severe knock set to a value of 45. It is recognized that thethreshold levels must be determined during engine characterization andare based on the knock tolerance level of the engine. The starting pointand size of the lower portion is determined by adjusting the startingpoint and size until the lower portion is of sufficient size and at alocation with respect to the ionization signal and crank angle such thatany portion of the ionization signal for an incipient knock conditionreliably falls within the lower portion 402 and remains outside theupper portion 404 as illustrated in FIG. 4 b. The starting point andsize of the upper portion is determined by adjusting the starting pointand size until the lower portion is of sufficient size and at a locationsuch that any portion of the ionization signal for a severe knockcondition reliably falls within the upper portion 404 as illustrated inFIG. 4 c.

In one embodiment, the starting point is a fixed amount of time afterthe ignition event and the duration of the floating box 400 is a fixedamount of time. This time can be in terms of actual time or in terms ofcrank angle and is determined from the data points illustrated in FIG. 4d. The ionization signal eventually goes to quasi steady state value.The bottom of the lower portion 402 is set to a point a fixed amountabove the quasi steady state value and the top of the lower portion 402is determined from the data points. The fixed amount above the quasisteady state value is determined from the data points and is set to alocation such that the ionization signals of normal combustionconditions do not fall within the floating box 400. The top of the lowerportion is determined such that incipient knock conditions fall withinthe lower portion 402 and remain outside the upper portion 404. Duringoperation, the quasi steady state value is determined and the lowerportion 402 of the floating box 400 is placed at the fixed amount oftime after the ignition event at the fixed amount above the quasi steadystate value. The ionization signal analyzer 132 provides an indicationto the ionization signal control module 134 that incipient knock hasbeen detected if the analyzer determines that the ionization signalfalls within the lower portion 402 while remaining outside the upperportion 404. The ionization signal analyzer 132 provides an indicationto the ionization signal control module 134 that severe knock has beendetected if the analyzer determines that the ionization signal fallswithin the upper portion 404.

As previously indicated, there are some spark plug configurations (andionization probe configurations) where the ionization signal does nothave a second peak. In systems having these types of configurations,knock is present when there is a large first peak in the ionizationsignal. Turning now to FIGS. 5 a-5 d, the floating bounded space 500 forconfigurations having no second peak is subdivided to include a lowerportion 502 and an upper portion 504 to detect the incipient knock andsevere knock. FIG. 5 a is an illustration of a representative cylinderpressure 506 and ionization signal 508 of a normal combustion condition.FIG. 5 b is an illustration of a representative cylinder pressure 510and ionization signal 512 for an incipient knock condition. FIG. 5 c isan illustration of a representative cylinder pressure 514 and ionizationsignal 516 for a severe knock condition. A representative set of datapoints of the engine parameter is shown in FIG. 5 d. Acceptable knock520, incipient knock 522, and severe knock 524 levels are shown. Theengine parameter used is the peak of the derivative of pressure of acylinder with respect to engine crank angle. A threshold level of thepeak of the derivative of pressure of a cylinder is chosen for incipientknock and a threshold level of the peak of the derivative of pressure ofa cylinder is chosen for severe knock. The threshold level for incipientknock is chosen such that the knock won't cause immediate damage to theengine. The threshold level for severe knock is chosen such that theknock is about to do some damage to the engine. If the peak of thederivative of pressure of a cylinder is below the threshold level forincipient knock, any knock present is within an acceptable level ofknock. If the peak of the derivative of pressure of a cylinder is abovethe threshold level for incipient knock and below the threshold forsevere knock, the knock is defined as incipient knock. If the peak ofthe derivative of pressure of a cylinder is above the threshold levelfor severe knock, the knock is defined as severe knock. In oneembodiment, the threshold level selected for incipient knock is set to avalue of 15 and the threshold level for severe knock is set to a valueof 45. Other values may be used. The starting point and size of thelower portion is determined by adjusting the starting point and sizeuntil the lower portion is of sufficient size and at a location withrespect to the ionization signal and crank angle such that any portionof the ionization signal for an incipient knock condition reliably fallswithin the lower portion 502 and remains outside the upper portion 504as illustrated in FIG. 5 b. The starting point and size of the upperportion is determined by adjusting the starting point and size until thelower portion is of sufficient size and at a location such that anyportion of the ionization signal for a severe knock condition reliablyfalls within the upper portion 504 as illustrated in FIG. 5 c.

In one embodiment, the starting point is a fixed amount of time afterthe ignition event and the duration of the floating box 500 is a fixedamount of time. This time can be in terms of actual time or in terms ofcrank angle and is determined from the data points illustrated in FIG. 5d. The ionization signal eventually goes to quasi steady state value.The bottom of the lower portion 502 is set to a point a fixed amountabove the quasi steady state value and the top of the lower portion 502is determined from the data points. The fixed amount above the quasisteady state value is determined from the data points and is set to alocation such that the ionization signals of normal combustionconditions do not fall within the floating box 500. The top of the lowerportion is determined such that incipient knock conditions fall withinthe lower portion 502 and remain outside the upper portion 504. Duringoperation, the quasi steady state value is determined and the lowerportion 502 of the floating box 500 is placed at the fixed amount oftime after the ignition event at the fixed amount above the quasi steadystate value. The ionization signal analyzer 132 provides an indicationto the ionization signal control module 134 that incipient knock hasbeen detected if the analyzer determines that the ionization signalfalls within the lower portion 502 while remaining outside the upperportion 504. The ionization signal analyzer 132 provides an indicationto the ionization signal control module 134 that severe knock has beendetected if the analyzer determines that the ionization signal fallswithin the upper portion 504.

The ionization signal control module 134 provides an indication to theair/fuel module 104 of the incipient knock conditions and the severeknock conditions and to other modules such as the engine controller. Theair/fuel module 104, EGR module 130, and spark module 106 (or the enginecontroller), in turn, determines what action to take. The actions thatcan be taken include retarding the ignition timing, running the engineleaner (e.g., adding more air or exhaust gas to the air/fuel mixture),or shutting down the engine. The air/fuel module 104 controls fuelinjection 124 and/or throttle valve 126, the EGR module 130 controls EGRvalve 128, and spark module 106 controls the spark timing to move theengine away from the knock condition by retarding the ignition timingand/or running the engine leaner, or shutting down the engine.

Turning now to FIG. 6, the steps of determining abnormal combustionconditions of a reciprocating engine having high EGR are shown. Whilethe steps will be described sequentially, it is recognized that thesteps may be performed sequentially, in parallel, a combination ofsequentially and parallel, and in different order. One or moreionization signals of the reciprocating engine for cycles (i.e.,combustion events) of a running engine is obtained (step 600).

The ionization signal is processed for signal stability and a resultantionization signal is determined (step 602). A start point of theresultant ionization signal and a peak for the resultant ionizationsignal is determined using an initial level for all of the signals (step604). The ionization signal is checked to determine if a portion of anionization signal is within a floating bounded space 300, 400, 500 (step606). An indication is provided if a portion of an ionization signal iswithin a floating bounded space 300, 400, 500 (step 608). The air/fuelmodule 104, EGR module 130, and spark module 106 (or the enginecontroller), in turn, determine what actions to take. The actions thatcan be taken include advancing or retarding the ignition timing, runningthe engine leaner or richer, or shutting down the engine. The air/fuelmodule 104 controls fuel injection 124 and/or throttle valve 126, EGRmodule 130 controls EGR valve 128, and spark module 106 controls thespark timing to move the engine away from the abnormal combustioncondition or shuts down the engine.

Turning to FIG. 7, step 606 includes determining if a portion of anionization signal is within floating bounded space 300 for the durationof the floating bounded space 300 (step 700). Some engines have sensorsor other engine performance indicators that can be used as a secondarysensor to verify that a misfire has occurred. If a portion of theionization signal is within the floating bounded space for an extendedinterval corresponding to the duration of the floating bounded space anda secondary sensor is available on the engine, the secondary sensor ischecked (step 702) to verify that a misfire has occurred. If available,the secondary sensor is used to eliminate the possibility that thatdrift of the flame kernel produced by the spark plug 120 has moved outof the spark gap 122 before the ionization module 102 has detected theionization signal. The secondary sensor is checked to see if the engineis operating normally. FIG. 8 illustrates the secondary sensor. Thesecondary sensor 190 provides a secondary signal 192 to the ionizationmodule 102. The secondary signal 192 may be a pressure signal 194,exhaust temperature 196, IMEP, instantaneous crank angle velocity 198,or other signals such as from an oxygen sensor and the like. If thesecondary signal provides confirmation that a misfire has occurred (step704), an indication of misfire is provided (step 608).

The ionization signal is also checked to determine if any portion of theionization signal falls within the lower portion 402, 502 floatingbounded space 400, 500, and no portion falls within the upper portion404, 504 of floating bounded space 400, 500 (step 706). If any portionof the ionization signal falls within the lower portion 402, 502floating bounded space 400, 500, and no portion falls within the upperportion 404, 504 of floating bounded space 400, 500, an indication ofincipient knock is provided (step 608).

The ionization signal is also checked to determine if any portion of theionization signal falls within upper portion 404, 504 of floatingbounded space 400, 500 (step 708). If any portion of the ionizationsignal falls within the upper portion 404, 504 of floating bounded space400, 500, an indication of severe knock is provided (step 608). Steps600-608 are repeated for subsequent combustion events.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

An apparatus and method to detect abnormal combustion conditions for usein a feedback control in a lean burn reciprocating engine usingionization signals has been described. Preferred embodiments of thisinvention are described herein, including the best mode known to theinventors for carrying out the invention. Variations of those preferredembodiments may become apparent to those of ordinary skill in the artupon reading the foregoing description. The inventors expect skilledartisans to employ such variations as appropriate, and the inventorsintend for the invention to be practiced otherwise than as specificallydescribed herein. Accordingly, this invention includes all modificationsand equivalents of the subject matter recited in the claims appendedhereto as permitted by applicable law. Moreover, any combination of theabove-described elements in all possible variations thereof isencompassed by the invention unless otherwise indicated herein orotherwise clearly contradicted by context.

1. A method for detecting an abnormal combustion condition in a sparkignited combustion chamber of a reciprocating engine having an exhaustgas recirculation (EGR) valve to recirculate exhaust gas to obtain adesired level of NOx emission, the abnormal combustion conditioncomprising one of a misfire and a knock, the method comprising the stepsof: detecting the variation of an ionization signal that changes withrespect to an engine parameter over a combustion event of thereciprocating engine; associating a floating bounded space with theionization signal; determining if a portion of the ionization signal iswithin the floating bounded space; and adjusting an output of the EGRvalve if the portion of the ionization signal is within the floatingbounded space.
 2. The method of claim 1 further comprising the step ofdetecting the ionization signal wherein the engine is operating with anair to fuel ratio corresponding to a λ greater than 1.4.
 3. The methodof claim 1 wherein the abnormal combustion condition is a misfire andthe step of determining if the portion of the ionization signal iswithin the floating bounded space comprises the step of determining ifthe portion of the ionization signal remains within the floating boundedspace for an extended interval corresponding to the duration of thefloating bounded space.
 4. The method of claim 3 further comprising thestep of confirming that the misfire has occurred by checking a secondarysensor.
 5. The method of claim 1 wherein the abnormal combustioncondition is knock and the step of determining if the portion of theionization signal is within the floating bounded space comprises thestep of determining if any portion of the ionization signal is withinthe floating bounded space.
 6. The method of claim 5 wherein thefloating bounded space comprises a first portion and a second portionand the step of determining if the portion of the ionization signal iswithin the floating bounded space comprises the step of determining ifany portion of the ionization signal is within one of the first portionand the second portion.
 7. The method of claim 6 wherein the step ofproviding the indication comprises the step of providing one of anindication of incipient knock if said any portion of the ionizationsignal is within the first portion and not the second portion and anindication of severe knock if said any portion of the ionization signalis within the second portion.
 8. The method of claim 1 furthercomprising the step of adjusting at least one of a position and size ofthe floating bounded space as a function of engine operating conditions,the engine operating conditions including at least one of an enginespeed, an engine load, and a desired percent EGR.
 9. The method of claim1 further comprising the step of adjusting a combustion parameter if theabnormal combustion condition has been detected.
 10. The method of claim9 wherein the abnormal engine condition is misfire and the step ofadjusting the combustion parameter comprises at least one of adjustingthe ignition timing and reducing the percent EGR.
 11. The method ofclaim 9 wherein the abnormal engine condition is knock and the step ofadjusting the combustion parameter comprises at least one of retardingthe ignition timing and adjusting the percent EGR.
 12. A method ofidentifying abnormal combustion cycles in a reciprocating engine havingan exhaust gas recirculation (EGR) valve to recirculate exhaust gas toobtain a desired level of NOx emission, the abnormal combustion cyclesbeing characterized by an abnormal event, the method comprising thesteps of: a) collecting ionization signals relating ionization currentto engine rotational position for a plurality of successive combustioncycles of the reciprocating engine, some of the combustion cycles beingnormal, and others of the combustion cycles being characterized by theabnormal event; b) identifying a characteristic of the ionization signalfor the abnormal combustion cycles which distinguishes from theionization signal for the normal combustion cycles; c) associating atleast one floating bounded space with the ionization signals andadjusting the position and size of the floating bounded space so thatthe floating bounded space captures the characteristic whichdistinguishes the abnormal combustion cycles; and d) testingsubsequently generated ionization signals with the floating boundedspace to distinguish between normal and abnormal combustion cycles ofthe reciprocating engine.
 13. The method of claim 12 wherein the engineis operating with an air to fuel ratio corresponding to a λ greater than1.4, the method further comprising the step of detecting the ionizationsignals.
 14. The method of claim 12 further including the steps ofidentifying a second characteristic of the ionization signal whichdistinguishes a second abnormal event from both the normal signal andthe abnormal event, and repeating steps c-d for the second abnormalevent.
 15. The method of claim 14 wherein the abnormal event isincipient knock and the second abnormal event is severe knock.
 16. Themethod of claim 12 wherein the abnormal event is one of misfire andknock.
 17. The method of claim 12 further comprising the step ofproviding an indication if an abnormal event is detected.
 18. The methodof claim 12 further comprising the step of adjusting an amount ofexhaust gas recirculating if an abnormal event is detected.
 19. Themethod of claim 12 wherein the step of adjusting the position and sizeof the floating bounded space includes adjusting at least one of theposition and the size of the floating bounded space as a function ofengine operating conditions, the engine operating conditions includingat least one of an engine speed, an engine load, and a desired amount ofexhaust gas recirculating in the reciprocating engine.
 20. The method ofclaim 12 wherein the abnormal event is misfire and the step ofassociating at least one floating bounded space with the ionizationsignals and adjusting the position and size of the floating boundedspace comprises the steps of: establishing a start engine rotationalposition; determining a duration of the floating bounded space;determining a lowest ionization signal level over the duration; andadjusting the position of the floating bounded space at the start enginerotational position to the lowest ionization signal level.
 21. Themethod of claim 12 further comprising the step of segregating theionization signals into ionization signals for normal combustion cyclesand ionization signals for abnormal combustion cycles based upon anengine parameter that can be used to identify whether the combustioncycle associated with an ionization signal is an abnormal combustioncycle or a normal combustion cycle.
 22. The method of claim 21 whereinthe engine parameter is indicated mean effective pressure.
 23. Themethod of claim 21 wherein the engine parameter is the peak of thederivative of cylinder pressure.
 24. A method to detect an abnormalcombustion condition of a reciprocating engine having an exhaust gasrecirculation (EGR) valve to recirculate exhaust gas to obtain a desiredlevel of NOx emission, the method comprising the steps of: associating afloating bounded space with an ionization signal such that the floatingbounded space captures a characteristic of the ionization signal whichdistinguishes the abnormal combustion condition from a normal combustioncondition for an engine operating; detecting the variation of anionization signal with respect to an engine parameter over a combustionevent; and adjusting the amount of exhaust gas recirculating if aportion of the ionization signal falls within the floating boundedspace.
 25. The method of claim 24 wherein the engine is operating withan air to fuel ratio corresponding to a λ greater than 1.4, the methodfurther comprising the step of detecting the ionization signal.
 26. Themethod of claim 24 wherein the abnormal combustion condition is amisfire and the step of adjusting the amount of exhaust gasrecirculating comprises the step of reducing the amount of exhaust gasrecirculating if the portion of the ionization signal remains within thefloating bounded space for an extended interval corresponding to theduration of the floating bounded space.
 27. The method of claim 24wherein the abnormal combustion condition is knock and the step ofadjusting the amount of exhaust gas recirculating comprises the step ofincreasing the amount of exhaust gas recirculating if any portion of theionization signal is within the floating bounded space.
 28. The methodof claim 24 wherein the floating bounded space comprises a first portionand a second portion, and the step of adjusting the amount of exhaustgas recirculating comprises the step of increasing the amount of exhaustgas recirculatingby a larger amount if any portion of the ionizationsignal is within the second portion than the amount increased when anyportion of the ionization signal is within the first portion and outsidethe second portion.
 29. A method to determine a floating bounded spaceand a starting point for the floating bounded space used to determine anabnormal combustion condition comprising the steps of: receiving a setof ionization signals that change with respect to an engine parameterover a combustion event, the set having ionization signals correspondingto normal combustion conditions and ionization signals corresponding toat least one abnormal combustion condition for an engine operating withan exhaust gas recirculation valve to recirculate exhaust gas to obtaina desired level of NOx emission; adjusting the starting point and a sizeof the floating bounded space such that selected portions of theionization signals corresponding to the at least one abnormal combustioncondition reliably fall within the floating bounded space and theionization signals corresponding to normal combustion conditionsreliably fall outside the floating bounded space.
 30. The method ofclaim 29 wherein the at least one abnormal combustion condition is amisfire and the step of adjusting the starting point and the sizecomprises the step of adjusting at least one of the starting point andthe region such that the selected portion of the ionization signalscorresponding to the at least one abnormal combustion condition reliablyremains within the floating bounded space for an extended intervalcorresponding to the duration of the floating bounded space and theionization signals corresponding to the normal combustion conditionsreliably fall outside of the floating bounded space.
 31. The method ofclaim 29 wherein the at least one abnormal combustion condition is aknock, the selected portion of the ionization signal is any portion ofthe ionization signal and the step of adjusting the at least one of thestarting point and the region comprises the step of adjusting at leastone of the starting point and the size such that the selected portion ofthe ionization signals corresponding to the at least one abnormalcombustion condition reliably falls within the floating bounded spaceand the ionization signals corresponding to the normal combustionconditions reliably fall outside of the floating bounded space.
 32. Themethod of claim 29 wherein the floating bounded space has an upperportion and a lower portion, the knock is an incipient knock and thestep of adjusting the at least one of the starting point and the sizecomprises the step of adjusting at least one of the starting point andthe size such that selected portion of the ionization signalscorresponding to the at least one abnormal combustion condition reliablyfalls within the lower portion and outside the upper portion and theionization signals corresponding to the normal combustion conditionsreliably fall outside the floating bounded space.
 33. The method ofclaim 29 wherein the floating bounded space has an upper portion and alower portion, the knock is an severe knock and the step of adjustingthe at least one of the starting point and the region comprises the stepof adjusting at least one of the starting point and the region such thatthe selected portion of the ionization signals corresponding to the atleast one abnormal combustion condition reliably falls within the upperportion and the ionization signals corresponding to the normalcombustion conditions reliably fall outside the upper portion.
 34. Themethod of claim 29 wherein the floating bounded space is a floating box.35. The method of claim 29 wherein the step of adjusting the startingpoint and the size of the floating bounded space includes adjusting atleast one of the starting point and the size of the floating boundedspace as a function of engine operating conditions, the engine operatingconditions including at least one of an engine speed, an engine load,and a desired percent of exhaust gas recirculating.
 36. A method todetect an abnormal combustion condition of a reciprocating engine havingan exhaust gas recirculation valve to recirculate exhaust gas to obtaina desired level of NOx emission, the method comprising the steps of:associating a floating bounded space with an ionization signal such thatthe floating bounded space captures a characteristic of the ionizationsignal which distinguishes the abnormal combustion condition from anormal combustion condition for an engine; detecting the variation of anionization signal with respect to an engine parameter over a combustionevent; detecting if a portion of the ionization signal falls within thefloating bounded space; and adjusting an amount of exhaust gasrecirculating if the portion of the ionization signal falls within thefloating bounded space.
 37. The method of claim 36 further comprisingthe step of providing an indication that the abnormal combustioncondition has been detected if the portion of the ionization signalfalls within the floating bounded space.
 38. The method of claim 36wherein the abnormal engine condition is misfire and the step ofadjusting the combustion parameter comprises at least one of adjustingthe ignition timing and reducing the amount of exhaust gasrecirculating.
 39. The method of claim 36 wherein the abnormal enginecondition is knock and the step of adjusting the combustion parametercomprises at least one of retarding the ignition timing and increasingthe amount of exhaust gas recirculating.